This invention relates to electroluminescent devices, and especially to controlling such devices.
One type of electroluminescent device is described in U.S. Pat. No. 5,247,190, the contents of which are incorporated herein by reference. The basic structure of this device is an organic light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)- "PPV") sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. The electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
Another type of organic light-emitting device is a small molecule device, details of which are given in U.S. Pat. No. 4,539,507. These have a light emitting layer of a small molecule material sandwiched between the two electrodes.
FIG. 1 shows a typical plot of light output against current arid voltage for a light-emitting polymer device. Such devices typically emit light at relatively low voltages (typical turn-on voltages are below 3V). This makes them suitable for simple battery-driven DC drive schemes. However, the devices often have relatively short lives because they degrade during use.
Intermittent drive schemes are used for LCD matrix displays. Typically, the electrode on one side of the display is patterned into rows and the electrode on the other side is patterned into columns. When a suitable voltage is applied to one row and one column light emission occurs where the two strips overlap. Thus, the points of overlap between the column strips and the row strips define the pixels of the display. A multiplexed drive scheme is often used, in which each row is addressed in turn (by scanning the row electrode strips sequentially) and the appropriate data is applied to the column electrode strips to apply data to the pixels of that row. Clearly, the functions of the rows and columns can be reversed; however, for clarity, in this document the convention described above will be maintained. The multiplexed scheme means that each pixel only receives a voltage for a fraction of the time that the display is in use. The fraction is related to the number of rows in the display: if the display has 500 rows then each pixel will be driven for 1/500 of the time that the display is in use.
U.S. Pat. No. 5,552,678 and Van Slyke et al., Appl. Phys. Lett. 69, 2160 (1996) have described the application of a basic AC drive scheme to an organic small molecule light-emitting device for increasing lifetime due to "catastrophic failure". The degradation mechanisms in the case of small molecule devices are very different from the degradation mechanisms in light emitting polymer devices, for instance because the cause of degradation in small molecules is that they are prongs to re-crystallisation, whereas polymers are not prone to re-crystallisation. Also, the chemical interactions of polymers and small molecules with interfaces in the device structure are very specific and different in the two cases. And the drive voltages used for polymer devices are generally much lower than those used for small molecule devices, which influences their degradation due to electric fields.
Heeks et al., Ferroelectrics 1991 vol. 27, p381 and Heeks et al., SID Digest 91 p404 have described other drive schemes in non-luminescent devices, for example ferroelectric liquid crystal displays. However, up to now there have been no proposals for advanced drive schemes for light-emitting polymer devices.
According to the present invention from one aspect there is provided a light-missive polymer device comprising: an anode; a cathode: a conjugated light-emissive polymer layer located between the anode and the cathode; and a driver for applying a voltage drive scheme between the anode and the cathode of a pattern having relatively high voltage portion which causes the polymer layer to emit light and a relatively low voltage portion during which substantially no light is emitted by the polymer layer.
As will be discussed in more detail below, the applicant has found that a drive scheme of this type has provided surprising increases in the lifetime of conjugated light-emissive polymer devices compared to conventional constant current or constant voltage drive schemes, for the same overall fight output.
According to the present invention from a second aspect there is provided an organic light-emissive device comprising: an anode; a cathode; an organic light-emissive layer located between the anode and the cathode; and a driver for applying a voltage drive scheme between the anode and the cathode of a pattern having a positive voltage portion which causes the organic layer to emit light, a negative voltage portion during which substantially no light is emitted by the organic layer, and a portion of substantially zero voltage during which substantially no light is emitted by the organic layer.
According to the present invention from a third aspect there is provided the use, for the purpose of extending the operating life time of a light-emissive polymer device having a light-emissive layer comprising a conjugated light-emissive polymer material, of a cyclical drive scheme in which a positive voltage sufficient to cause the polymer layer to emit light is applied for only part of the cycle.
The relatively high voltage portion suitably provides a positive voltage. The voltage of the relatively high voltage portion may suitably be at least 1.5, 2, 2.5 or 3 times the turn-on voltage of the polymer layer, preferably in the range from 1.01 to 100 times the turn-on voltage of the polymer layer and most preferably in the range from 2 to 8 times the turn-on voltage. The turn-on voltage is the lowest positive voltage at which the layer emits light. The relatively high voltage portion is preferably greater than 4V, and most preferably greater than 6V. The voltage is preferably sufficient for the device to appear to the eye of a user to give a substantially continuous light output. The relatively high positive voltage is preferably in the range from 1 to 200V.
The off time during the drive scheme cycle is preferably sufficiently short for the device to appear to the eye of a user to give a substantially continuous light output.
The relatively low voltage portion may comprise a portion (in particular an non-instantaneous portion) of zero or substantially zero voltage and/or a portion of negative voltage. The relatively low voltage portion may comprise two or more constant voltage portions each of different voltages. At least one of these voltages is suitably a negative voltage. The amplitude of the negative voltage may be greater than that of the positive voltage, suitably at least 1.5, 2 or 3 times the amplitude of the positive voltage. Alternatively the amplitude of the negative voltage may be less than that of the positive voltage, in which case the amplitude of the positive voltage is suitably at least 3, 4 or 5 times the amplitude of the negative voltage. The amplitude of the negative voltage is suitably in the range from 0.1V to 200V, preferably from 1V to 200V.
The duration of the relatively low voltage portion (and especially any negative voltage portion of that) may be greater than or less than the duration of the relatively high voltage portion. The durations of the portions and their voltage profiles may be selected so that the integrated voltage of the drive scheme over time is zero or substantially zero and/or so that the integrated injected current of the drive scheme over time is zero or substantially zero and/or so that the integrated injected power of the drive scheme over time is minimised or substantially minimised. Instead of selecting any or all of these parameters to be zero they could be minimised or substantially minimised whilst maintaining adequate light output from the device and/or apparently continuous light output from the device and/or whilst maximising the expected time to failure of the device. The drive scheme preferably comprises repeating a cycle of the relatively high and relatively low voltages portions. Any or all of the parameters mentioned above are preferably set to zero or substantially zero or minimised over one such cycle.
One or both of the anode and the cathode is preferably continually connected to the drive means throughout the drive scheme. Alternatively, at least one of them could be periodically disconnected from the drive means, as during the multiplexing of a matrix display.
The drive scheme is preferably cyclical. It preferably comprises repeating a cycle of the positive, negative and substantially zero voltage portions. The cycle preferably includes a negative voltage portion immediately followed bat a positive voltage portion. The cycle preferably includes a positive voltage portion immediately followed by a portion of substantially zero voltage immediately followed by a positive voltage portion. The cycle preferably includes a negative voltage portion immediately followed by a portion of substantially zero voltage immediately followed by a negative voltage portion. The cycle preferably includes a first positive voltage portion immediately followed by a negative voltage portion, and a second positive voltage portion immediately preceded by a negative voltage portion. Alternatively, the drive scheme may comprise a random or pseudo-random series of pulses.
The device may also comprise a drive means for applying a further positive voltage between the anode and the cathode of a voltage less than the turn-on voltage of the polymer light-missive layer.
According to the present invention from a fourth aspect there is provided a light-emissive polymer device comprising: an anode; a cathode; a conjugated light-emissive polymer layer located between the anode and the cathode; and a driver for applying a voltage drive scheme between the anode and the cathode of a pattern having a first, preconditioning positive voltage portion of an amplitude that is insufficient to cause the polymer layer to emit light and a second, activating voltage portion of an amplitude that is sufficient to cause the polymer layer to emit light.
The preconditioning voltage portion is preferably applied before the activating voltage portion.
The organic light-emissive layer preferably comprises a luminescent material. The luminescent material is suitably a conjugated material. The organic light-emissive layer suitably comprises one or more of a conjugated polymer, an organometallic light-emissive material and a non-polymeric material such as a luminescent small molecule (see for example U.S. Pat. No. 4,539,507).
Polymeric light-emissive materials suitable for use according to this invention especially include conductive or semi-conductive polymers, for instance semiconductive conjugated polymer materials. The polymer layer preferably comprises one or more luminescent polymers, such as PPV, poly(2-methoxy-5(2'-ethyl)hexyloxyphenylene-viylene) ("MEH-PPV"), polyfluorenes, polyparaphenylenes, etc., or co-polymers thereof, and including substituted and/or unsubstituted versions thereof. It could be deposited by spin-coating, dip-coating, blade-coating, meniscus-coating self-assembly etc. The constituent of the light-emissive layer and/or its precursor is preferably soluble, most preferably in water: examples are precursor-based PPV derivatives such as MEH-PPV. The thickness of the layer is preferably in at least the range from 10 to 200 nm and most preferably around 1000 .ANG..
The term "conjugated" indicates a material which is partially, mostly, fully or substantially fully conjugated. It includes within its scope homopolymers and copolymers.
Suitable small molecule materials include tris(8-hydroxyquineleto)aluminium (alq.sub.3) and derivatives thereof.
The anode is preferably of a high work function material, suitably with a work function greater that 4 eV and most preferably greater than 4.5 eV. The layer is preferably comprised of ITO, or a metal or alloy (e.g. gold) or a suitable conducting material such as e.g. a conducting polymer such as polystyrene sulphonic acid doped polyethylene dioxythiophene ("PEDT/PSS"), polyaniline, a doped alkylthiophene or a doped polypyrrole.
The cathode is preferably of a low work function material, suitably with a work function less than 3.5 eV and most preferably less than 3 eV. The layer may comprise a metal or alloy such as aluminium, lithium and/or calcium. The layer is preferably deposited by for example sputtering or evaporation. The cathode may be provided with a layer of a suitable electron transport material, such as polyoxadiazole, to lie between the cathode and the light-missive layer.
The device preferably comprises a conductive polymer layer between the light-emissive layer and one of the electrodes, especially if the electrode is based on indium tin oxide (ITO). The conductive polymer layer preferably is or comprises polyethylene dioxythiophene ("PEDT"), PEDT/PSS, doped polyaniline, a doped alkylthiophene and/or a doped polypyrrole. The thickness of the layer is suitably less than 200 nm, preferably less than 100 nm and most preferably less than or around 50 nm.
According to a further aspect of the present invention there is provided a method of driving an organic luminescent device by applying a drive scheme as described herein.